The present invention concerns a rocker lever for the valve operation of an internal combustion engine to actuate the opening of a valve stem against the spring force of a valve spring which closes the valve, being outfitted with a mechanical valve clearance adjusting element.
A typical internal combustion engine uses an arrangement of valves to control the intake and exhausting of gases into and out of an engine cylinder. The opening and closing of a valve normally occurs electromagnetically or purely mechanically in that the valve stem is activated by a lever arrangement, which in turn is controlled by a camshaft driven by the crankshaft of the engine. As a rule, the downwardly directed movement of the lever arrangement and the associated activating of the end of the valve stem involves a lifting of the valve head from its seat. This movement is counteracted by the restoring force of an appropriately arranged valve spring, which makes sure that the valve head is pulled back into its tight engagement with the valve seat at the proper time in the engine cycle.
In order to prevent tolerances in the valve operation and temperature fluctuations during the operating of the internal combustion engine from resulting in hard knocking together of valve mechanism and valve stem and also to ensure the secure closing of the valve, it is known how to provide a valve clearance of predetermined (minimum) dimension between valve mechanism and valve stem. However, due to wear and tear, this decreases in size during the operation of the engine, which leads to unwanted changes in the precise valve control required for the operation of the engine and possibly further damage resulting from this.
For this reason, the valve clearance must be adjusted/readjusted to a predetermined magnitude after a corresponding operating time of the engine, which generally involves considerable costs and lengthy down time. Furthermore, this maintenance work requires appropriate technical knowledge and a certain amount of equipment.
One solution might be self-acting systems for automatic adjustment/readjustment of the valve clearance, such as are already known. Very widespread is an automatic hydraulic valve clearance equalization (DE 2200131 C2), in which a pressure space between a cylinder and a piston which can travel therein can be filled with pressurized oil via a check valve, so that the piston interacting with the end of the valve stem can equalize the valve clearance at any time.
However, such hydraulic systems have the drawback that they depend on a supply of pressurized oil from the lubrication system and their response time is dependent on the buildup in oil pressure and the oil viscosity. Furthermore, they are complicated in construction and relatively costly to produce, so that their use in simple internal combustion engines is unsuited.
Mechanically acting valve clearance equalizing devices are also known. DE 43 39 433 A1 describes a valve lever with a deflection device, comprising an adjustment wedge having two wedge surfaces, which is arranged between a supporting wedge secured in position in the direction of adjustment of the adjusting wedge in the valve lever and an adjusting wedge of a valve pressing piece which can move in the direction of the gas exchange valve. With this device, it is possible to substantially reduce the area pressure between the valve pressing piece and its guidance in the valve lever. Also, in EP 0 331 901 A2, a mechanical valve clearance equalization is described in which a rocker interacts with an eccentric disk to eliminate the valve clearance. The eccentricity of the eccentric disk is chosen so that the transmission ratio of the rocker is reduced during the valve actuating stroke, but after the valve returns to its closing position and the transmission forces of the spring disappear, the eccentric disk swivels until the valve clearance is eliminated.
On the other hand, it is desirable to have a purely mechanical and automatically functioning device for adjusting/readjusting the valve clearance, one which is easy and economical to produce.
This purpose is accomplished by a rocker according to the construction of the present invention. According to the invention, the valve clearance adjusting element mentioned in the introduction is characterized in that it comprises a hollow cylinder, placed and able to turn in the rocker, with flat surfaces (a xe2x80x9csliding blockxe2x80x9d) formed about the circumference of the hollow cylinder, and a torsion spring which turns the sliding block in a predetermined direction of rotation. Adjacent flat surfaces are bounded off from each other by their surface edges.
The torsion spring for turning the sliding block in a predetermined direction of rotation has a much lower spring force than the valve spring which closes the valve, so that the closing of the valve experiences no impairment from a contrary directed force component of the torsion spring.
According to the invention, the sliding block is configured such that the flat surfaces formed about the circumference of the hollow cylinder are each arranged parallel to the axis of the hollow cylinder. This implies that there is a perpendicular to the hollow cylinder axis for each flat surface. This perpendicular coincides in its direction with a radial direction of the circular cross section of the hollow cylinder. According to the invention, the perpendicular through a flat surface to the hollow cylinder axis defines the shortest distance between the flat surface and the hollow cylinder axis. The flat surfaces have surface edges which are parallel to the hollow cylinder axis (and thus also parallel to each other). In relation to the turning of the sliding block in a predetermined direction as produced by the torsion spring, the surface edges of each flat surface that are parallel to the axis of the hollow cylinder are to be distinguished as a leading forward surface edge and a trailing rear surface edge.
The sliding block is characterized in that its flat surfaces have increasing shortest distances from the cylinder axis in the direction opposite the predetermined direction of rotation, as well as an increasing radial distance of the corresponding rear surface edges from the cylinder axis.
According to the invention, furthermore, the flat surfaces each end up in a rotary position to activate the valve stem due to the automatic rotation of the sliding block produced by the torsion spring. In the closing position of the valve and the corresponding position of the rocker, a first predetermined valve clearance results from the difference between the perpendicular distance of the axis of rotation of the hollow cylinder from the end of the valve stem and the shortest distance of the flat surface from the cylinder axis for each rotary position of a flat surface. In the rotary position to actuate the valve stem, within the range of a given valve clearance for each flat surface, spontaneous further turning of the sliding block is precluded by the bearing of the corresponding rear surface edge of the flat surface against the end of the valve stem. Thus, when a rear surface edge is bearing against the end of the valve stem, the spring force of the torsion spring does not produce any turning of the sliding block, but instead a pretensioning of the sliding block in the direction of turning.
In the closing position of the valve and the corresponding position of the rocker, the flat surface is slanted in the position to activate the valve stem by virtue of the torsion spring applying the corresponding rear surface edge against the end of the valve stem. When the valve stem is activated by the tilting of the rocker, this slanting of the flat surfaces with respect to the ends of the valve stem is eliminated and the flat surface then bears fully against the end of the valve stem.
The valve clearance adjusted in this way has a tendency to become enlarged with wear and tear, so that when the valve clearance becomes large enough, the rear surface edge of the flat surface loses its purchase against the end of the valve stem in the rotary position to actuate the valve stem, and then the automatic turning of the sliding block by virtue of the spring force of the torsion spring commences. Of course, the turning can only commence if the flat surface in the position of actuating the valve stem does not bear fully against the end of the valve stem, i.e., only if there exists the appropriately increased valve clearance in the closing position of the valve and the corresponding position of the rocker.
According to the invention, the automatic turning of the sliding block by abutment of the rear surface edge belonging to the flat surface in the position of activating the valve stem is prevented until such time as the valve clearance attains a second predetermined valve clearance, corresponding to the difference between the radial distance of the rear surface edge of the flat surface from the cylinder axis and the shortest distance of this flat surface from the cylinder axis. Wearing of the rear surface edge, which is to be expected in practice will, however, shorten the readjustment interval. Since the perpendicular distance of the rotational axis of the hollow cylinder from the end of the valve stem for a particular position of the rocker lever in the closing position of the valve is essentially constant, it is possible to adjust or readjust a first predetermined valve clearance in terms of the increasing shortest distance of the flat surfaces from the cylinder axis, because each flat surface ends up in the rotary position to activate the valve stem by the automatic turning of the sliding block.
In the basic setting of the sliding block, it is adjusted so that the flat surface ends up in a rotary position to activate the valve stem with the smallest shortest distance from the axis of the hollow cylinder. This adjusts the first predetermined valve clearance. Preferably, the rocker lever is configured such that the torsion spring is a twice-supported helical spring arranged about the axis of the hollow cylinder, having a rigid bearing on the sliding block and the other rigid bearing on the rocker. This simplifies the construction, saves space and is economical.
An advantageous embodiment of the invention calls for the rocker lever to be outfitted with at least four flat surfaces, so that the adjustment/readjustment of the first predetermined valve clearance can be accomplished at least four times.
Furthermore, it is preferable that the first predetermined valve clearance be identical for each flat surface in its rotary position to activate the valve stem. This achieves a repeated readjustment of the valve clearance to the identical predetermined value. In particular, for example, a constant minimum valve clearance can be adjusted for each readjustment stage.
Likewise, it is preferable that the second predetermined valve clearance be identical for each flat surface in its rotary position to activate the valve stem. This ensures that a particular valve clearance which is always the same for each flat surface in the rotary position to active the valve stem is not exceeded. In combination with a constant first predetermined valve clearance, the valve clearance can thus be kept within the bounds defined by the first and second predetermined valve clearance.
In preferred fashion, the first predetermined valve clearance lies in the range of 0.01 mm to 1 mm; preferably, this is 0.1 mm. In preferred manner, the second predetermined valve clearance has a difference from the first predetermined valve clearance which lies in the range of 0.05 mm to 1 mm; preferably, this is 0.2 mm.
An advantageous embodiment of the invention specifies that the flat surfaces of the sliding block are bounded on either side by shim disks mounted on the hollow cylinder. In this way, the sliding block can be easily and at the same time effectively secured in its rotary mounting in the rocker lever as regards its deflection in the direction of the rotary axis, without impairing the automatic turning of the sliding block.
Furthermore, according to the invention, it is preferable that the second predetermined valve clearance of the flat surface with the largest shortest distance, i.e., that flat surface which is the last to engage in the rotary position for activating the valve stem, be chosen large enough to ensure the bearing of the rear surface edge for an average operating life of the internal combustion engine. In this way, the flat surface with the largest shortest distance from the axis of the hollow cylinder, i.e., the one with the maximum ability to equalize the wear and tear over time, will be permanently utilized for the remaining lifetime of the engine.
Preferably, the sliding block is made from chill casting, sintered material, cold-pressed or extruded molded steel.
As already mentioned, the device according to the present invention is easy and economical to produce. However, a special advantage of the invention is that it requires very little room and is almost universally applicable. Moreover, already fabricated internal combustion engines which employ a rocker lever can be easily and economically retrofitted by replacing the current rocker lever with that of the invention.